Voice Activity Detection

Voice Activity Detection

In a phone call, normally one party speaks while the other one listens to the speaker. As a
standard PSTN link provides 64 kbps full duplex channels, half of the capacity is usually
wasted during this kind of communication.

Voice over IP makes these resources available for other purposes if Voice Activity Detection
is activated.

How does this work? VAD detects the loudness of the speaker's voice and decides when to
stop speech frame packetizing. Before doing so, VAD waits for a fixed period of time, mostly
200 ms. In very noisy environments, VAD might have problems distinguishing between
speech and background noise. At the start of the call a defined signal-to-noise ratio, also
called the signal threshold, is used to decide whether to automatically activate or de-activate
the VAD operations.

If the VAD procedure detects the absence of voice and simply switches off speech
information transmission to the distant party, they might think that the call has been
interrupted. In order to avoid this, the systems inserts an artificial noise, called Comfort
Noise, to make the listening party believe that the link is still present.